image-responsiveness

I am making login panel for students. Just like any other sites, I am providing option to upload profile pic. As you guys know, user can select image of any resolution. But we cant save as it is. So i just want upload that user-entered-image in particular resolution like (ex. 150*150) so that it will be responsive according to screen resolution. But solution must be through code of HTML or CSS. Any help would be appreciated. Thank you.